I'm not entirely sure, but it doesn't sound like the Yellow Ribbon Program would apply to you through your fiance. The moment you're no longer the dependent of your step-father you stop receiving your aid.
EDIT:
I did a little more research and actually read about the program which is just an extension of the GI BIll (they improved it in 01') Your step dad's benefits would no longer be applied to you- however if you married your fiance and he has served at least 36 months active duty he can sign his GI benefits over to you (which is what your step dad is doing).
I understand you probably want to get married, but sit down and talk to your fiance and figure out what he wants to do with his GI Bill money.. he may want to go back to school or he can use it while he serves to supplement his TA (tuition assistance). If you do choose to get married there are huge benefits like healthcare, BAH (if he lives off base, etc)